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2 GHz tunable superconducting band-pass filter using a piezoelectric bender
Physica C 366 (2002) 183±189
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2 GHz tunable superconducting band-pass ®lter using a piezoelectric bender Y. Terashima *, H. Fuke, F. Aiga, M. Yamazaki, H. Kayano, R. Kato Corporate Research and Development Center, Toshiba Corporation, 1 Komukai Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan Received 26 April 2001; received in revised form 22 May 2001; accepted 22 May 2001
Abstract We have developed a 2 GHz tunable superconducting band-pass ®lter using a piezoelectric bending actuator. The ®lter is a 12-pole microstrip line structure on 50 mm diameter LaAlO3 substrate and has low loss and sharp skirt characteristics. The center frequency was 1.94 GHz and the attenuation characteristics were 30 dB at 1 and 3 MHz apart from the lower and higher band-edge, respectively. A sapphire (Al2 O3 ) plate was stacked on the ®lter with an air gap and the gap thickness was controlled electrically from the outside of a refrigerator using a piezoelectric bending actuator. The center frequency of the ®lter could be adjusted within 11 MHz in such a manner that the low loss and sharp skirt characteristics were not degraded. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: High-Tc superconductor; Filter; Tunable; Piezoelectric actuator; Dielectric material
1. Introduction High-Tc superconducting microwave ®lters have low loss and sharp skirt characteristics and have been expected to be applied to the receiving systems of mobile telecommunication base stations [1±4]. However, the sharp skirt characteristic demands strict fabrication tolerances and design procedures to fabricate a ®lter with the desired characteristics and it is often necessary to tune the center frequency of the band-pass ®lter after it has been manufactured, because the center frequency of the ®lter may vary from the design value due to
* Corresponding author. Tel.: +81-44-549-2110; fax: +81-44520-1801. E-mail address: [email protected] (Y. Terashima).
the variation in permittivity of dielectric substrate or thickness of substrate [5]. The ability to tune the center frequency from the outside of a refrigerator is also desired, because temperature ¯uctuation over time causes the shift of the center frequency. Moreover, ®lters whose center frequency can be electrically tuned from the outside of the refrigerator are considered to be an important element of the rf front-end of the new mobile communication systems. Tuning of the center frequency has been predominantly achieved by magnetic [6±9], electric [5,10±12] and mechanical tuning methods [4,13, 14]. In the magnetic tuning, a ferrite material such as Y3 Fe5 O12 (YIG) near the ®lter is DC magnetized to change the resonance frequency. However, the use of tunable devices incorporating ferrites has so far been limited due to large size and high insertion loss at frequencies lower than 10 GHz.
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 8 8 8 - 7
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The electrical tuning methods utilize the dependence of permittivity of dielectric material on electric ®eld. Currently, SrTiO3 and (BaSr)TiO3 have been used for the dielectric materials. However, they have been found to seriously degrade the Q-factor of the resonators that compose the ®lter and deteriorate the ®lter characteristics such as insertion loss and skirt characteristic. On the other hand, the mechanical tuning is achieved by means such as tuning screws, and the insertion loss is not large. However, it is not easy to tune the frequency of the superconducting ®lter by this means, because the ®lter is placed in a vacuum vessel and is cooled down to cryogenic temperature. Moreover, many screws are necessary and the ®lter's package becomes large if the ®lter consists of many resonators in order to get a sharp skirt characteristic. Thus, it is desirable to be able to tune the center frequency of the superconducting ®lter from the outside of a refrigerator in such a manner that the low loss and sharp skirt characteristics are not deteriorated and the ®lter package does not become large. In this paper, we describe a novel tunable superconducting band-pass ®lter with low loss and sharp skirt characteristics. The ®lter is of the 12pole microstrip line type on a 50 mm diameter LaAlO3 substrate. To tune the center frequency of the ®lter, a sapphire (Al2 O3 ) plate is stacked on the ®lter with an air gap and the gap thickness is controlled electrically from the outside of a refrigerator using a piezoelectric bending actuator.
2. Experimental Fig. 1 shows a layout of a 12-pole band-pass ®lter, which consists of twelve half-wavelength microstrip transmission line resonators. YBa2 Cu3 Oy (YBCO) ®lms were deposited on both sides of a 50 mm diameter, 0.5 mm thick LaAlO3 (LAO) substrate and one of the YBCO ®lms was patterned by photolithography and the Ar dry etching technique. The ®lter pattern was designed using the moment method under the condition that no Al2 O3 plate for tuning the center frequency was stacked. The designed center frequency and
Fig. 1. Schematic illustration of a 12-pole band-pass ®lter.
bandwidth were 1.93 GHz and 25 MHz, respectively. A 1 mm thick Al2 O3 plate was stacked on the above ®lter with an air gap and the ®ltering characteristics were measured at 60 K, changing the thickness of the air gap. The air gap thickness was controlled by (1) inserting four 2 2 mm2 spacers between the dielectric Al2 O3 plate and the ®lter or by (2) moving the dielectric plate with a piezoelectric bending actuator. The ®lter using the latter method is the tunable ®lter in the present work. The plane ®gure (a) and the cross-sectional schematic (b) of the tunable ®lter are shown in Fig. 2. The bimorph-piezoelectric bending actuator consists of two thin PZT plates with parallel polarization and is 58 mm long, 10 mm wide and 0.5 mm thick. One end of a 6 mm diameter and 10 mm long Al2 O3 rod is glued to the bender and another end is glued to the center of a 30 mm wide, 27 mm long and 1 mm thick dielectric Al2 O3 plate using a low-temperature epoxy, ``Stycast-1266''. Sapphire was used for the dielectric plate and the rod because the dielectric loss is very small and 1 mm thick of the plate was necessary in order to neglect the in-plane distribution of the eective dielectric constant due to the existence of the epoxy layer and the Al2 O3 rod. To achieve a translational movement of the Al2 O3 plate, both sides near the ends of the bender are ®xed on pieces of machinable glass ceramic, Macor, that are connected to the base plate on which the 12-pole ®lter is settled and the Al2 O3 rod was glued at the center of the bender. Attention was paid to mount the bender so as not to hinder some tilt or rotation of the PZT
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with the ®lter, and create a suitable air gap between the Al2 O3 plate and the ®lter surface, when DC voltage is applied to the bender. The voltage dierence between a center electrode and surface electrodes to control the air gap can swing between 300 and 300 V. The maximum displacement (stroke) of the bender is 1000 lm for 300 V at room temperature under the condition that one side is ®xed and one side moves freely. When it is mounted on both sides near the ends to get a pure translational movement, the mid-elongation is 1=4 of the oneside free moving arrangement. Moreover, strokes of the PZT benders at 60 K are about 30% of room temperature value. Therefore, the stroke of the Al2 O3 plate in the present tunable ®lter is estimated to be 75 lm for 300 V. 3. Results and discussion 3.1. Eects of a dielectric plate on a transmission characteristic of the ®lter Fig. 2. Structure of a tunable superconducting band-pass ®lter using a piezoelectric bending actuator: (a) plane ®gure and (b) cross-sectional schematic.
layer at the support points. Concretely, both ends of the bender were clamped with two pairs of halfcylindrical Macor pieces using screws and springs as shown in Fig. 2(b). The parallel of the Al2 O3 plate to the ®lter surface was adjusted by the two epoxy-layers, when we set up the bender, the Al2 O3 rod and the Al2 O3 plate under the condition that the distance between the Al2 O3 plate and a plate-spacer on the ®lter was zero. After such mounting, the plate-spacer was removed. Thickness of the air gap at cryogenic temperature would dier from that at room temperature, the thickness of plate-spacer. Therefore, we estimated the initial thickness of the air gap at cryogenic temperature by referring the relation between the frequency shift of the ®lter and the thickness of the air gap such as Fig. 4, before applying the voltage to the electrode of the bender. In this way, the Al2 O3 plate covering the ®lter can move in a direction perpendicular to the ®lter plane, keeping parallel
Fig. 3 shows microwave transmission S21 versus frequency for several air gap lengths that are determined by the thickness of the spacers and Fig. 4 shows a gap-length dependence of the center frequency, bandwidth (BW) and ripple (R). In Fig. 3, the S21 parameter for the ®lter without an Al2 O3 plate (bare ®lter) is also shown, which was the same as that for the ®lter with an air gap of
Fig. 3. Measured frequency responses at 60 K under various air gaps between an Al2 O3 plate and a ®lter element.
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Fig. 4. Center frequency, bandwidth and ripple as a function of an air gap between an Al2 O3 plate and a ®lter element.
Fig. 5. Simulated results for 12-pole ®lter with an Al2 O3 plate at 0.25 mm and without the Al2 O3 plate.
1.0 mm. The center frequency and the bandwidth of the bare ®lter were 1.946 GHz and 29 MHz, respectively, and attenuation characteristics were 30 dB=1.0 MHz and 30 dB=3.0 MHz at lower and higher edge of pass-band, respectively. As shown in these ®gures, the center frequency shifted to the lower frequency up to 25.6 MHz with the decrease in the gap length. At gap lengths larger than 0.25 mm, where the center frequency shifts were less than about 10 MHz, sharp skirt and low loss characteristics were not degraded. Ripple and bandwidth, however, changed slightly, that is, increase in the ripple of less than 0.2 dB and decrease in the bandwidth of less than 1.9 MHz. At gap lengths smaller than 0.2 mm, or at the frequency shifts larger than 10 MHz, however, the transmission characteristics at both edges of pass-band were clearly degraded. The change in the center frequency results from the change of the eective dielectric constant around the ®lter. In the present experiment, eective dielectric constant becomes large with the decrease in an air gap between a ®lter and an Al2 O3 plate whose dielectric constant is about 10, and the center frequency shifts to lower frequency. In general, it is considered that the change of the eective dielectric constant leads also to the changes of the coupling strengths between resonant elements and external Q-values, and that causes the changes in skirt characteristic, insertion loss, ripple and bandwidth. However, the above results showed that when frequency shift was
smaller than about 10 MHz, the changes in the skirt characteristic, insertion loss and ripple were small. The result of an electromagnetic simulation for the same structure as that used in the experiment with an air gap of 0.25 mm is shown in Fig. 5. In the ®gure, the result for the bare ®lter is also shown. Fig. 5 shows the frequency shift of 10 MHz, decrease in the bandwidth of 1.5 MHz and no change in the skirt characteristics and the ripple that is less than 0.2 dB, when the Al2 O3 plate is stacked with an air gap of 0.25 mm. Ref. [13] describes that though the variations in the frequencies of the resonators composing a ®lter aect the characteristics in pass-band such as the ripple, the eects of variation in the coupling strengths on the in-band characteristics are small. This also explains the small change in pass-band characteristics with a change of the center frequency in the present experiment. The change in bandwidth with a tuning of the center frequency was small, but may not be negligible depending on the application system. In that case, tuning of the bandwidth is also necessary. The tuning of the bandwidth will be possible by connecting two ®lters in series and controlling the center frequency of each ®lter independently. In our above-mentioned experiment using four spacers of the same thickness, the Al2 O3 plate was parallel to the ®lter and the variation in the frequencies of the resonators due to the tilt of the plate was negligible. The S21 parameters for the
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Fig. 6. Frequency responses at 60 K (a) without and (b) with tilt of an Al2 O3 plate.
®lter stacked with an Al2 O3 plate declined (b), and not declined (a) at 0.3 mm from the ®lter surface are shown in Fig. 6. When the Al2 O3 plate was stacked with a tilt of 0:7° in a direction from an input port to an output port, the S21 characteristic changed considerably as shown by (b) in Fig. 5. Therefore, when a dielectric plate is mechanically moved to control the center frequency of the ®lter, attention has to be paid in order to keep parallel to the ®lter plane. In summary of the above discussion, it will be possible to tune the center frequency of the ®lter without degrading the sharp skirt and low loss characteristics by mechanically moving an Al2 O3 plate under the condition that the Al2 O3 plate and a ®lter substrate are parallel. 3.2. Frequency tuning using a piezoelectric bender Fig. 7 shows (a) S21 parameters and (b) the characteristics in pass-band, such as the center frequency (fc), bandwidth (BW) and ripple (R), of the tunable ®lter using an Al2 O3 plate and a piezoelectric bending actuator for several voltages applied to the piezoelectric bender at 60 K. The distance between the ®lter and the Al2 O3 plate before applying the voltage was 0.4 mm. The DC voltage was swept from 300 to 300 V, decreasing the air gap length. The center frequency modulations were within 5 MHz depending on the applied voltage of 300 V and, in that range,
Fig. 7. Transmission characteristics of the tunable ®lter for various voltages applied to the bender at 60 K with an initial air gap of 0.4 mm: (a) S21 parameters and (b) center frequency, bandwidth and ripple.
sharp skirt and low loss characteristics were not degraded. Referring to Fig. 4, stroke of 75 lm of the Al2 O3 plate leads to the center frequency modulation of 3±5 MHz when the air gap is about 0.4 mm, which coincides with the result shown in Fig. 7. The changes in the ripple and the bandwidth with the tuning of the center frequency were small, 0.6 dB and 0.9 MHz, respectively. Considering the experimental variations of the center frequency, a tunability of 5±10 MHz is desired. In the following, the initial distance between the Al2 O3 plate and the ®lter is decreased. Fig. 8(a) shows S21 parameters when the DC voltage was swept from 300 to 300 V and Fig. 8(b) shows the characteristics in pass-band when the DC voltage was swept from 0 V through 300 and 300 to 0 V. The initial distance between the Al2 O3 plate and the ®lter was 0.25 mm. The center frequency tunability was 11.4 MHz, which is twice
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due to the hysteric motion of the bender, and the center frequencies at increasing voltage are dierent from that at decreasing voltage. Thus, in order to control the center frequency of the ®lter with applying voltage only, it is necessary to apply a maximum or minimum voltage to the bender at ®rst, and next to apply a voltage for a desired frequency. 4. Conclusion
Fig. 8. Transmission characteristics of the tunable ®lter for various voltages applied to the bender at 60 K with an initial air gap of 0.25 mm: (a) S21 parameters and (b) center frequency, bandwidth and ripple.
that of shown in Fig. 7. However, when applied voltages were <0 V and the center frequencies were less than 1.935 GHz, the transmission characteristics at both edges of pass-band changed and the ripple increased signi®cantly as shown in Fig. 8(b). These results also coincide with those shown in Figs. 3 and 4, that is, when the frequency shift from that of ®lter without the Al2 O3 plate is larger than about 10 MHz, the transmission characteristics at both edges of pass-band changed considerably. Although the center frequency of the ®lter is determined with the distance between the Al2 O3 plate and the ®lter, it is not easy to measure the distance, and it is more practical to control the center frequency by the applied voltage only. The relation between the center frequency and the applied voltage is shown in Fig. 8(b). The center frequency does not depend on applied voltage only
We have fabricated a tunable 12-pole superconducting band-pass ®lter with sharp skirt and low loss characteristics, using a 50 mm diameter LaAlO3 substrate. The center frequency and the bandwidth were 1.94 GHz and 29 MHz, respectively. By stacking an Al2 O3 plate over the ®lter with an air gap, and by controlling the gap thickness with a piezoelectric bending actuator, the center frequency of the ®lter could be shifted electrically within 11 MHz without the degradation of the low loss and sharp skirt characteristics. A novel method to tune the center frequency of the superconducting band-pass ®lter has been presented. This method might provide great promise for tunable ®lters because the center frequency can be adjusted electrically after packaging the ®lter in a compact size. Acknowledgements The authors would like to thank Dr. N. Gemma for his encouragement. References [1] G. Tsuzuki, M. Suzuki, N. Sakakibara, IEEE MTT-S Int. Symp. Dig. (2000) 669. [2] E.R. Soares, K.F. Raihn, A.A. Davis, R.L. Alvarez, P.J. Marozick, G.L. Hey-Shipton, IEEE Trans. Appl. Supercond. 9 (1999) 4018. [3] R.B. Greed, D.C. Voyce, D. Jedamzik, J.S. Hong, M.J. Lancaster, M. Reppel, H.J. Chaloupka, J.C. Mage, B. Marcilhac, R. Mistry, H.U. Hafner, G. Auger, W. Rebernak, IEEE Trans. Appl. Supercond. 9 (1999) 4002. [4] M. Reppel, H. Chaloupka, IEEE MTT-S Int. Symp. Dig. (1999) 1563.
Y. Terashima et al. / Physica C 366 (2002) 183±189 [5] H. Fuke, Y. Terashima, H. Kayano, M. Yamazaki, F. Aiga, R. Kato, IEEE Trans. Appl. Supercond. 11 (2001) 434. [6] G.F. Dionne, D.E. Oates, IEEE Trans. Magn. 33 (1997) 3421. [7] G.F. Dionne, D.E. Oates, D.H. Temme, J.A. Weiss, IEEE Trans. Microwave Theory Tech. 44 (1996) 1361. [8] D.E. Oates, G.F. Dionne, IEEE Trans. Appl. Supercond. 9 (1999) 4170. [9] T. Fukusako, M. Tsutsumi, IEEE Trans. Microwave Theory Tech. 45 (1997) 2013.
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[10] A.T. Findikoglu, Q.X. Jia, X.D. Wu, G.J. Chen, T. Venkatesan, D.W. Reagor, Appl. Phys. Lett. 68 (1996) 1651. [11] G. Subramanyam, F. Van Kesuls, F.A. Miranda, IEEE MTT-S Int. Symp. Dig. (1998) 1011. [12] D.C. Degroot, J.A. Beall, R.B. Marks, D.A. Rudman, IEEE Trans. Appl. Supercond. 5 (1995) 2272. [13] J.S. Hong, M.J. Lancaster, D. Jedamzik, R.B. Greed, IEEE Trans. Microwave Theory Tech. 47 (1999) 1656. [14] E.R. Soares, IEEE MTT-S Int. Symp. Dig. (1999) 1555.
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2 GHz tunable superconducting band-pass ®lter using a piezoelectric bender Y. Terashima *, H. Fuke, F. Aiga, M. Yamazaki, H. Kayano, R. Kato Corporate Research and Development Center, Toshiba Corporation, 1 Komukai Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan Received 26 April 2001; received in revised form 22 May 2001; accepted 22 May 2001
Abstract We have developed a 2 GHz tunable superconducting band-pass ®lter using a piezoelectric bending actuator. The ®lter is a 12-pole microstrip line structure on 50 mm diameter LaAlO3 substrate and has low loss and sharp skirt characteristics. The center frequency was 1.94 GHz and the attenuation characteristics were 30 dB at 1 and 3 MHz apart from the lower and higher band-edge, respectively. A sapphire (Al2 O3 ) plate was stacked on the ®lter with an air gap and the gap thickness was controlled electrically from the outside of a refrigerator using a piezoelectric bending actuator. The center frequency of the ®lter could be adjusted within 11 MHz in such a manner that the low loss and sharp skirt characteristics were not degraded. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: High-Tc superconductor; Filter; Tunable; Piezoelectric actuator; Dielectric material
1. Introduction High-Tc superconducting microwave ®lters have low loss and sharp skirt characteristics and have been expected to be applied to the receiving systems of mobile telecommunication base stations [1±4]. However, the sharp skirt characteristic demands strict fabrication tolerances and design procedures to fabricate a ®lter with the desired characteristics and it is often necessary to tune the center frequency of the band-pass ®lter after it has been manufactured, because the center frequency of the ®lter may vary from the design value due to
* Corresponding author. Tel.: +81-44-549-2110; fax: +81-44520-1801. E-mail address: [email protected] (Y. Terashima).
the variation in permittivity of dielectric substrate or thickness of substrate [5]. The ability to tune the center frequency from the outside of a refrigerator is also desired, because temperature ¯uctuation over time causes the shift of the center frequency. Moreover, ®lters whose center frequency can be electrically tuned from the outside of the refrigerator are considered to be an important element of the rf front-end of the new mobile communication systems. Tuning of the center frequency has been predominantly achieved by magnetic [6±9], electric [5,10±12] and mechanical tuning methods [4,13, 14]. In the magnetic tuning, a ferrite material such as Y3 Fe5 O12 (YIG) near the ®lter is DC magnetized to change the resonance frequency. However, the use of tunable devices incorporating ferrites has so far been limited due to large size and high insertion loss at frequencies lower than 10 GHz.
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 8 8 8 - 7
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The electrical tuning methods utilize the dependence of permittivity of dielectric material on electric ®eld. Currently, SrTiO3 and (BaSr)TiO3 have been used for the dielectric materials. However, they have been found to seriously degrade the Q-factor of the resonators that compose the ®lter and deteriorate the ®lter characteristics such as insertion loss and skirt characteristic. On the other hand, the mechanical tuning is achieved by means such as tuning screws, and the insertion loss is not large. However, it is not easy to tune the frequency of the superconducting ®lter by this means, because the ®lter is placed in a vacuum vessel and is cooled down to cryogenic temperature. Moreover, many screws are necessary and the ®lter's package becomes large if the ®lter consists of many resonators in order to get a sharp skirt characteristic. Thus, it is desirable to be able to tune the center frequency of the superconducting ®lter from the outside of a refrigerator in such a manner that the low loss and sharp skirt characteristics are not deteriorated and the ®lter package does not become large. In this paper, we describe a novel tunable superconducting band-pass ®lter with low loss and sharp skirt characteristics. The ®lter is of the 12pole microstrip line type on a 50 mm diameter LaAlO3 substrate. To tune the center frequency of the ®lter, a sapphire (Al2 O3 ) plate is stacked on the ®lter with an air gap and the gap thickness is controlled electrically from the outside of a refrigerator using a piezoelectric bending actuator.
2. Experimental Fig. 1 shows a layout of a 12-pole band-pass ®lter, which consists of twelve half-wavelength microstrip transmission line resonators. YBa2 Cu3 Oy (YBCO) ®lms were deposited on both sides of a 50 mm diameter, 0.5 mm thick LaAlO3 (LAO) substrate and one of the YBCO ®lms was patterned by photolithography and the Ar dry etching technique. The ®lter pattern was designed using the moment method under the condition that no Al2 O3 plate for tuning the center frequency was stacked. The designed center frequency and
Fig. 1. Schematic illustration of a 12-pole band-pass ®lter.
bandwidth were 1.93 GHz and 25 MHz, respectively. A 1 mm thick Al2 O3 plate was stacked on the above ®lter with an air gap and the ®ltering characteristics were measured at 60 K, changing the thickness of the air gap. The air gap thickness was controlled by (1) inserting four 2 2 mm2 spacers between the dielectric Al2 O3 plate and the ®lter or by (2) moving the dielectric plate with a piezoelectric bending actuator. The ®lter using the latter method is the tunable ®lter in the present work. The plane ®gure (a) and the cross-sectional schematic (b) of the tunable ®lter are shown in Fig. 2. The bimorph-piezoelectric bending actuator consists of two thin PZT plates with parallel polarization and is 58 mm long, 10 mm wide and 0.5 mm thick. One end of a 6 mm diameter and 10 mm long Al2 O3 rod is glued to the bender and another end is glued to the center of a 30 mm wide, 27 mm long and 1 mm thick dielectric Al2 O3 plate using a low-temperature epoxy, ``Stycast-1266''. Sapphire was used for the dielectric plate and the rod because the dielectric loss is very small and 1 mm thick of the plate was necessary in order to neglect the in-plane distribution of the eective dielectric constant due to the existence of the epoxy layer and the Al2 O3 rod. To achieve a translational movement of the Al2 O3 plate, both sides near the ends of the bender are ®xed on pieces of machinable glass ceramic, Macor, that are connected to the base plate on which the 12-pole ®lter is settled and the Al2 O3 rod was glued at the center of the bender. Attention was paid to mount the bender so as not to hinder some tilt or rotation of the PZT
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with the ®lter, and create a suitable air gap between the Al2 O3 plate and the ®lter surface, when DC voltage is applied to the bender. The voltage dierence between a center electrode and surface electrodes to control the air gap can swing between 300 and 300 V. The maximum displacement (stroke) of the bender is 1000 lm for 300 V at room temperature under the condition that one side is ®xed and one side moves freely. When it is mounted on both sides near the ends to get a pure translational movement, the mid-elongation is 1=4 of the oneside free moving arrangement. Moreover, strokes of the PZT benders at 60 K are about 30% of room temperature value. Therefore, the stroke of the Al2 O3 plate in the present tunable ®lter is estimated to be 75 lm for 300 V. 3. Results and discussion 3.1. Eects of a dielectric plate on a transmission characteristic of the ®lter Fig. 2. Structure of a tunable superconducting band-pass ®lter using a piezoelectric bending actuator: (a) plane ®gure and (b) cross-sectional schematic.
layer at the support points. Concretely, both ends of the bender were clamped with two pairs of halfcylindrical Macor pieces using screws and springs as shown in Fig. 2(b). The parallel of the Al2 O3 plate to the ®lter surface was adjusted by the two epoxy-layers, when we set up the bender, the Al2 O3 rod and the Al2 O3 plate under the condition that the distance between the Al2 O3 plate and a plate-spacer on the ®lter was zero. After such mounting, the plate-spacer was removed. Thickness of the air gap at cryogenic temperature would dier from that at room temperature, the thickness of plate-spacer. Therefore, we estimated the initial thickness of the air gap at cryogenic temperature by referring the relation between the frequency shift of the ®lter and the thickness of the air gap such as Fig. 4, before applying the voltage to the electrode of the bender. In this way, the Al2 O3 plate covering the ®lter can move in a direction perpendicular to the ®lter plane, keeping parallel
Fig. 3 shows microwave transmission S21 versus frequency for several air gap lengths that are determined by the thickness of the spacers and Fig. 4 shows a gap-length dependence of the center frequency, bandwidth (BW) and ripple (R). In Fig. 3, the S21 parameter for the ®lter without an Al2 O3 plate (bare ®lter) is also shown, which was the same as that for the ®lter with an air gap of
Fig. 3. Measured frequency responses at 60 K under various air gaps between an Al2 O3 plate and a ®lter element.
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Fig. 4. Center frequency, bandwidth and ripple as a function of an air gap between an Al2 O3 plate and a ®lter element.
Fig. 5. Simulated results for 12-pole ®lter with an Al2 O3 plate at 0.25 mm and without the Al2 O3 plate.
1.0 mm. The center frequency and the bandwidth of the bare ®lter were 1.946 GHz and 29 MHz, respectively, and attenuation characteristics were 30 dB=1.0 MHz and 30 dB=3.0 MHz at lower and higher edge of pass-band, respectively. As shown in these ®gures, the center frequency shifted to the lower frequency up to 25.6 MHz with the decrease in the gap length. At gap lengths larger than 0.25 mm, where the center frequency shifts were less than about 10 MHz, sharp skirt and low loss characteristics were not degraded. Ripple and bandwidth, however, changed slightly, that is, increase in the ripple of less than 0.2 dB and decrease in the bandwidth of less than 1.9 MHz. At gap lengths smaller than 0.2 mm, or at the frequency shifts larger than 10 MHz, however, the transmission characteristics at both edges of pass-band were clearly degraded. The change in the center frequency results from the change of the eective dielectric constant around the ®lter. In the present experiment, eective dielectric constant becomes large with the decrease in an air gap between a ®lter and an Al2 O3 plate whose dielectric constant is about 10, and the center frequency shifts to lower frequency. In general, it is considered that the change of the eective dielectric constant leads also to the changes of the coupling strengths between resonant elements and external Q-values, and that causes the changes in skirt characteristic, insertion loss, ripple and bandwidth. However, the above results showed that when frequency shift was
smaller than about 10 MHz, the changes in the skirt characteristic, insertion loss and ripple were small. The result of an electromagnetic simulation for the same structure as that used in the experiment with an air gap of 0.25 mm is shown in Fig. 5. In the ®gure, the result for the bare ®lter is also shown. Fig. 5 shows the frequency shift of 10 MHz, decrease in the bandwidth of 1.5 MHz and no change in the skirt characteristics and the ripple that is less than 0.2 dB, when the Al2 O3 plate is stacked with an air gap of 0.25 mm. Ref. [13] describes that though the variations in the frequencies of the resonators composing a ®lter aect the characteristics in pass-band such as the ripple, the eects of variation in the coupling strengths on the in-band characteristics are small. This also explains the small change in pass-band characteristics with a change of the center frequency in the present experiment. The change in bandwidth with a tuning of the center frequency was small, but may not be negligible depending on the application system. In that case, tuning of the bandwidth is also necessary. The tuning of the bandwidth will be possible by connecting two ®lters in series and controlling the center frequency of each ®lter independently. In our above-mentioned experiment using four spacers of the same thickness, the Al2 O3 plate was parallel to the ®lter and the variation in the frequencies of the resonators due to the tilt of the plate was negligible. The S21 parameters for the
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Fig. 6. Frequency responses at 60 K (a) without and (b) with tilt of an Al2 O3 plate.
®lter stacked with an Al2 O3 plate declined (b), and not declined (a) at 0.3 mm from the ®lter surface are shown in Fig. 6. When the Al2 O3 plate was stacked with a tilt of 0:7° in a direction from an input port to an output port, the S21 characteristic changed considerably as shown by (b) in Fig. 5. Therefore, when a dielectric plate is mechanically moved to control the center frequency of the ®lter, attention has to be paid in order to keep parallel to the ®lter plane. In summary of the above discussion, it will be possible to tune the center frequency of the ®lter without degrading the sharp skirt and low loss characteristics by mechanically moving an Al2 O3 plate under the condition that the Al2 O3 plate and a ®lter substrate are parallel. 3.2. Frequency tuning using a piezoelectric bender Fig. 7 shows (a) S21 parameters and (b) the characteristics in pass-band, such as the center frequency (fc), bandwidth (BW) and ripple (R), of the tunable ®lter using an Al2 O3 plate and a piezoelectric bending actuator for several voltages applied to the piezoelectric bender at 60 K. The distance between the ®lter and the Al2 O3 plate before applying the voltage was 0.4 mm. The DC voltage was swept from 300 to 300 V, decreasing the air gap length. The center frequency modulations were within 5 MHz depending on the applied voltage of 300 V and, in that range,
Fig. 7. Transmission characteristics of the tunable ®lter for various voltages applied to the bender at 60 K with an initial air gap of 0.4 mm: (a) S21 parameters and (b) center frequency, bandwidth and ripple.
sharp skirt and low loss characteristics were not degraded. Referring to Fig. 4, stroke of 75 lm of the Al2 O3 plate leads to the center frequency modulation of 3±5 MHz when the air gap is about 0.4 mm, which coincides with the result shown in Fig. 7. The changes in the ripple and the bandwidth with the tuning of the center frequency were small, 0.6 dB and 0.9 MHz, respectively. Considering the experimental variations of the center frequency, a tunability of 5±10 MHz is desired. In the following, the initial distance between the Al2 O3 plate and the ®lter is decreased. Fig. 8(a) shows S21 parameters when the DC voltage was swept from 300 to 300 V and Fig. 8(b) shows the characteristics in pass-band when the DC voltage was swept from 0 V through 300 and 300 to 0 V. The initial distance between the Al2 O3 plate and the ®lter was 0.25 mm. The center frequency tunability was 11.4 MHz, which is twice
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due to the hysteric motion of the bender, and the center frequencies at increasing voltage are dierent from that at decreasing voltage. Thus, in order to control the center frequency of the ®lter with applying voltage only, it is necessary to apply a maximum or minimum voltage to the bender at ®rst, and next to apply a voltage for a desired frequency. 4. Conclusion
Fig. 8. Transmission characteristics of the tunable ®lter for various voltages applied to the bender at 60 K with an initial air gap of 0.25 mm: (a) S21 parameters and (b) center frequency, bandwidth and ripple.
that of shown in Fig. 7. However, when applied voltages were <0 V and the center frequencies were less than 1.935 GHz, the transmission characteristics at both edges of pass-band changed and the ripple increased signi®cantly as shown in Fig. 8(b). These results also coincide with those shown in Figs. 3 and 4, that is, when the frequency shift from that of ®lter without the Al2 O3 plate is larger than about 10 MHz, the transmission characteristics at both edges of pass-band changed considerably. Although the center frequency of the ®lter is determined with the distance between the Al2 O3 plate and the ®lter, it is not easy to measure the distance, and it is more practical to control the center frequency by the applied voltage only. The relation between the center frequency and the applied voltage is shown in Fig. 8(b). The center frequency does not depend on applied voltage only
We have fabricated a tunable 12-pole superconducting band-pass ®lter with sharp skirt and low loss characteristics, using a 50 mm diameter LaAlO3 substrate. The center frequency and the bandwidth were 1.94 GHz and 29 MHz, respectively. By stacking an Al2 O3 plate over the ®lter with an air gap, and by controlling the gap thickness with a piezoelectric bending actuator, the center frequency of the ®lter could be shifted electrically within 11 MHz without the degradation of the low loss and sharp skirt characteristics. A novel method to tune the center frequency of the superconducting band-pass ®lter has been presented. This method might provide great promise for tunable ®lters because the center frequency can be adjusted electrically after packaging the ®lter in a compact size. Acknowledgements The authors would like to thank Dr. N. Gemma for his encouragement. References [1] G. Tsuzuki, M. Suzuki, N. Sakakibara, IEEE MTT-S Int. Symp. Dig. (2000) 669. [2] E.R. Soares, K.F. Raihn, A.A. Davis, R.L. Alvarez, P.J. Marozick, G.L. Hey-Shipton, IEEE Trans. Appl. Supercond. 9 (1999) 4018. [3] R.B. Greed, D.C. Voyce, D. Jedamzik, J.S. Hong, M.J. Lancaster, M. Reppel, H.J. Chaloupka, J.C. Mage, B. Marcilhac, R. Mistry, H.U. Hafner, G. Auger, W. Rebernak, IEEE Trans. Appl. Supercond. 9 (1999) 4002. [4] M. Reppel, H. Chaloupka, IEEE MTT-S Int. Symp. Dig. (1999) 1563.
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